A couple of people have sort of hinted at this, but there is limited markup available for comments. There is more information on this here - meta.stackexchange.com/questions/63286/… - but a really useful tip is to click on the help link under Add Comment, as this will provide you with a full list. This is a great posts/wiki by the way.
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Aaron NewtonDec 5 '11 at 21:48

Would it be wise, to allow trusted websites such as JS fiddle, to paste their iframe? e.g: <iframe width="100%" height="300" src="http://jsfiddle.net/fD5u3/embedded/" allowfullscreen="allowfullscreen" frameborder="0"></iframe>
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ValMar 24 '14 at 21:43

6

Is there a reason that <u> is not on this list?
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user259718Apr 29 '14 at 8:32

Note that since we allow the above HTML tags, to have tags show up as text you need to escape the < character. You can escape '<' by marking it as code (with indentation or backticks as is done below) or by using the HTML entity &lt;

Allowed Attributes

img Attributes

The following attributes are allowed on the <img> tag, but note that the mobile theme enforces a maximum width of 90%, so specifying a height might not scale the image proportionally on the mobile sites.

The attribute order is important! Using a different order (e.g., height before width) will strip the tag!

src=""
width="" (up to 999; do not include the 'px' extension)
height="" (up to 999; do not include the 'px' extension; see note above)
alt=""
title=""

a Attributes

The following attributes are allowed on the <a> tag:

href=""
title=""

Important Notes

HTML tags unlisted above are stripped from the output. They may render in the client preview, but they will always be removed on the server.

You must enter the tags exactly as shown. Any deviation from this list—adding extra spaces, using single quote or no quotes, etc.—means the tag will be stripped.

We do not (and will not) allow <table> tags. Sorry. This is intentional and by design. If you need a quick and dirty "table", use <pre> and ASCII layout.